This invention relates to the design of the structures and equipment supporting and surrounding the upper portion of water cooled nuclear reactors. The accident analysis required by the Nuclear Regulatory Commission (NRC) on nuclear plant designs includes the LOCA (Loss of Coolant Accident). The LOCA is defined as an instantaneous, circumferential, guillotine severance of the main coolant loop piping at selected locations. The primary loop equipment must be designed to survive the LOCA from the standpoint to core cooling. Specifically, those components which contribute to safety injection, control rod tripping and maintaining the mechanical configuration of the core may not exceed "Faulted Stress Limits" as defined by the NRC and the ASME Code. One of the main coolant loop piping locations requiring LOCA consideration is the weld joint between the reactor vessel nozzle and the nozzle safe end. The nozzle safe end is a stainless steel ring welded to the carbon steel nozzle in the vessel shop. Its purpose is to preclude the necessity of performing a bi-metallic weld in the field when joining the main coolant piping to the reactor vessel nozzles. Consideration of a LOCA at the nozzle safe end location predicts a pressurization of the space between the reactor vessel and the primary shield in the vicinity of the nozzle which is assumed to fail. This pressurization of the reactor cavity causes an asymmetric loading against the reactor vessel and its support systems. This condition results in stresses in the reactor vessel support shoes which could exceed the faulted stress limits.
Consequently it is desired to provide a reactor cavity arrangement which will reduce pressurization effects and resulting asymmetric forces on the reactor vessel.